I'm using hardie board - same price (currently) as Plywood, weights a ton, hard on your tools, but doesn't rot, get termited, or otherwise break down in much. (Its basically fiberglass reinforced concrete board.
But if price (approx $35/ 4x8 panel, or roughly $1.10/sq ft) is the factor you are trying to avoid, then go Craigslist and similar for "free near me", look for scrap lumber, used sheetmetal, pallets you can harvest, etc. I've been shopping the big box stores pretty extensively, and there is no outdoor suitable product I can think of which you can currently buy under $1/ sq ft except the dog eared fence pickets at about $.70/ sq ft, and then only if you get pine. Which should be painted or stained for weather resistance. Cedar pickets are roughly twice the price.